Multiple access communication systems are designed to provide access to limited communication resources, such as a channel, by a plurality of communication terminals for the purpose of exchanging communication messages, referred to variously as packets, data packets, messages, etc., between an infrastructure or network and the terminal. The access methodology, referred to as a multiple access protocol, is chosen such that some appropriate set of performance constraints are met. Typical performance constraints include efficiency of communication resource use, communication message delay, and other similar factors.
Multiple access protocols can generally be regarded as belonging to one of two types, contention and non-contention. Non-contention protocols are designed such that a terminal desiring to send a packet is permitted exclusive use of a communication resource or channel. One example of this type of protocol is time-division multiple access (TDMA) where the communication resource or channel is divided into a plurality of time frames that are further subdivided into a plurality of time slots and each terminal is assigned exclusive use of one or more time slots in each time frame.
This type of protocol is inefficient for terminals that infrequently source or generate messages since the assigned time slot is idle or unused by anyone most of the time. The practical number of terminals that can be accommodated by such a protocol is also limited by the delay incurred while waiting for one's assigned slot. This wait usually increases proportionally to the number of terminals that have assigned slots.
Contention protocols are characterized by terminals that actively compete or contend with each other to gain access to the communication resource or channel. The slotted ALOHA protocol is an example of this type of protocol. In slotted ALOHA, a communication resource or channel is divided into a plurality of time slots. A terminal desiring to send a packet may transmit in the first subsequent time slot, taking care not to transmit outside of the boundaries of that time slot. If no other terminal also transmitted in that same time slot, the packet transmission is considered successful. Note that other factors, such as communication channel noise, may ultimately result in a failure of the message, but that these other factors are not related to the access protocol. If one or more other terminals, however, did transmit a packet in the same time slot, all transmissions, ignoring capture and the like effects would fail due to collision. Thus contention protocols generally work well for lightly loaded systems, but performance suffers as load increases because the likelihood of collisions also increases. Further, communication messages longer than the time slot duration must be sent in a plurality of time slots and are subject to collision in each time slot used.
Reservation protocols, a sub-class of contention protocols, are also known. Reservation protocols attempt to combine certain aspects of contention and non-contention protocols to provide improved performance for a wider variety of communication system conditions. A typical reservation protocol divides a communication resource or channel into a series of fixed-size time frames further divided into a series of time slots. The time slots are comprised of two types, a reservation time slot and a data time slot, with varying numbers of each in each time frame. The reservation time slots are often significantly smaller than the data time slots and are typically grouped together at the beginning of each time frame.
A terminal desiring access to the communication resource or channel contends or, in pure ALOHA, transmits randomly in one of the reservation time slots for the purpose of reserving an associated data time slot. If the unit successfully avoids collision or successfully contends i.e. is the only unit to transmit a reservation request in a given reservation time slot, it is permitted exclusive access to an assigned or associated data time slot occurring later in the time frame. Although reservation protocols improve the effectiveness by which a communication resource or channel may be utilized by a plurality of competing terminals, particularly when the reservation requests are significantly shorter than the average message lengths, some drawbacks exist with these schemes.
A communication unit or terminal desiring access to the channel must first wait for the reservation time slots. If no messages are currently being sent, this represents a delay which would not have occurred if the terminal had been allowed to transmit immediately, in random access fashion. Further delay is encountered between the time the terminal successfully accesses the reservation time slot, via random access, and the time it receives confirmation of its reservation.
Additionally, even with the reservation protocols all terminals must still contend for the uplink or inbound channel under some conditions. To further improve this aspect various contention management algorithms have been proposed and used. In particular Carrier Sense Multiple Access (CSMA) is a contention management scheme where the terminals seeking to access the channel must first confirm that the channel is idle, i.e. not busy prior to requesting access. In circumstances where the terminals do not see, that is can not monitor, the channel that they are contending for, such as many wireless communications systems, a technique known as Digital Sense Multiple Access (DSMA) is often used. In this approach the infrastructure monitors the contended for channel and communicates information corresponding to the state of the channel, such as busy or idle, to the terminals on the downlink or outbound channel. Note that some method of feedback to the terminal regarding the success or failure of access requests is necessary in order for these protocols to be effective.
Another known embellishment to access protocols is a contention management mechanism that includes a random time delay having a normal distribution with a fixed upper bound. This random delay is initiated prior to an access attempt under circumstances where multiple terminals will be contending with virtual certainty for the channel. This upper bound on the random time delay is a design constant for all terminals in a given communication system. A contending terminal looks for the channel idle indication and transmits, a packet or access request packet or alternatively sees a channel busy indication and waits a random time prior to looking again. In other words, the terminal waits a random time before transmitting and this reduces the probability of a collision with a transmission from another terminal.
While the random delay is an effective contention management mechanism or process under some circumstances, it fails when either the channel is heavily or lightly loaded and under various other circumstances, such as load differentiation or management, encountered in state of the art systems. If such circumstances are encountered the results may be suboptimum or inefficient use of the channel or long delay times between transmissions. For example, if the number of terminals drops in a given system, the terminals contending for a channel may wait an unnecessarily long time to transmit. Since the number of terminals had decreased, the probability of transmissions colliding also had decreased. However, the terminals will continue to wait the random times dictated by the fixed design constant and therefore not allow efficient utilization of the channel resources. On the other hand, if the number of users becomes significantly large, transmission collisions will become more frequent as the terminals contend for channel access. Unless the probability values used by the terminals is such that no collisions could be avoided by choosing different probability values, the system is not operating at the most efficient level.
Accordingly, there is a need for an adaptive channel access technique which will enable the efficient utilization of communication resources.